High temperature reactors are presently employed to carry out pyrolysis, thermolysis, dissociation, decomposition and combustion reactions of both organic and inorganic compounds. Substantially all such reactors transfer heat to the reactants by convection and/or conduction, but this characteristic inherently produces two major problems which limit the nature and scope of the reactions which may be carried out. Both problems result from the fact that in a conventional reactor which transfers heat to the reactants by convection, the highest temperature in the system is necessarily at the interface between the inside wall of the reactor and the reactant stream.
The first problem involves the limitations on available temperatures of reaction which are imposed by the strength at elevated temperatures of known reactor wall materials. The decreasing capability of such materials to maintain their integrity under conditions of increasing temperature is, of course, well known. However, since it is necessary that such materials be heated in order that thermal energy may be transferred to the reactant stream, available reaction temperatures have been limited by the temperature to which the reactor wall could be safely heated. This factor is particularly critical in cases where the contemplated reaction either must take place at or produces high pressures.
The second problem inherently results both because the wall of a conventional reactor is at the highest temperature in the system and because convective/conductive heat transfer requires contact between the wall and the reactant stream. Being at such elevated temperature, the reactor wall is an ideal if not the most desirable reaction site in the system and, in many instances, reaction products will accumulate and build up on the wall. Such build-up impairs the ability of the system to transfer heat to the reactants and this ever increasing thermal impedance requires the source temperature to be raised progressively just to maintain the initial rate of heat transfer into the reactant stream. Obviously, as the build-up increases, the required source temperature will eventually exceed the capabilities of the reactor wall material. Moreover, as additional energy is required to sustain the reaction, the process becomes less efficient in both the technical and economic sense. Thus, at the point where the contemplated reaction can no longer be sustained on the basis of either heat transfer, strength of materials, or economic considerations, the system must be shut down and cleaned.
Usually, cleaning is performed mechanically by scraping the reactor wall or chemically by burning off the deposits. In some continuous processes, it has been attempted to scrape the reactor wall while the reaction proceeds. However, the scraping tool itself necessarily gets hot, becomes a reaction site and, thereafter, must be cleaned. In any event, this down time represents a substantial economic loss. In many instances, a second system will be installed in order to minimize lost production time. However, such additional equipment generally represents a substantial capital investment. Some high temperature chemical reactors include a tube which is heated to a temperature at which its inner walls emit sufficient radiant energy to initiate and sustain the reaction. However, as in the case of conductive and convective reactors, for reactions yielding solid products there is frequently an undesirable build-up of product on the tube walls which leads to reduced heat transfer and even clogging of the tube.
The reactor disclosed in U.S. Pat. No. 2,926,073 is designed to produce carbon black and hydrogen by the pyrolysis of natural gas. The process is stated to be continuous but, in practice, the convective heat transfer principle under which the reactor operates causes serious problems both in sustaining and controlling the reaction. Since the heated tubes of the reactor are ideal reaction sites, carbon invariably builds up and eventually clogs the system. More serious, however, is the problem of thermal runaway which can result in explosions. With respect to this condition, it has been determined that during pyrolysis of natural gas, thermal conductivity of the gas phase suddenly increases from about five to thirty times, depending upon the composition of the gas. Because the temperatures in a conventional convective reactor cannot be regulated with sufficient speed and accuracy to compensate for this phenomenon, in some instances the system would become unstable and explosions would result. Such conditions are inherent in conventional reactors and, as yet, no way has been found to overcome this problem.
U.S. Pat. No. 3,565,766 represents a recent attempt to upgrade coal by pyrolysis. The disclosed system comprises a series of hollow steel vessels which act as a multi-stage fluidized beds at successively increasing temperatures up to about 1600.degree. F. Fluidization at lower temperatures is achieved by an inert gas which may itself supply heat although external heating is contemplated. At higher temperatures, fluidization is achieved by the overhead gas obtained in the final stage; and, in the final stage, temperature is maintained by internal combustion of the char in air or oxygen. Because it relies primarily upon heat transfer by convection, this system is subject to many of the defects and disadvantages which have previously been discussed.
The apparatus for the manufacture of carbon black disclosed in U.S. Pat. No. 2,062,358 includes a porous tube disposed within a heating chamber. Hot gas is directed from a remote furnace into the chamber, and thereafter forced through the wall of the porous tube to mix with the reactants. Thus, only convective transfer of heat from a fluid to reactants is employed. This, together with the absence of a "black body cavity" necessitates the flow of a large volume of fluid through the heating chamber in order to make up for heat losses.
U.S. Pat. No. 2,769,772 discloses a reactor for heat-treating fluid materials such as hydrocarbons which includes two concentric tubes disposed in a flame heated furnace. Reactants flow axially through the pervious inner concentric tube. A heat-carrier gas flowing in the annular chamber between the concentric tubes is heated by contact with the outer wall. Fluids in the inner tube are heated by convection when the heat-carrier gas passes through the pervious wall and mixes with them. Radiant heat transfer is expressly avoided. In fact, it is impossible to heat the inner tube without simultaneously heating the outer tube to at least as high a temperature.
The surface-combustion cracking furnace of U.S. Pat. No. 2,436,282 employs the convective heat carrier gas principle similar to that of U.S. Pat. No. 2,769,772. The furnace includes a porous, refractory tube enclosed by a jacket. A combustible fluid from an annular chamber is forced through the porous wall to the inside of the tube where it is ignited. It is evident, however, that the combustible fluid in the annular chamber will explode unless it is forced through porous wall at a rate faster than the rate of flame propagation back through the wall. Likewise, the temperature in the annular chamber must be maintained below the ignition temperature of the gas/air mixture. Combustion products from the surface flame mix with reactants in the furnace diluting and possibly reacting with them. Heat is imparted to the reactants by convective mixing of the combustion products and the reactants.
U.S. Pat. Nos. 2,670,272; 2,670,275; 2,750,260; 2,915,367; 2,957,753; and 3,499,730 disclose combustion chambers for producing pigmentary titanium dioxide by burning titanium tetrachloride in oxygen. In the U.S. Pat. No. 2,670,275 patent which is representative of this group, titanium tetrachloride is burned in a porous, refractory tube. An inert gas is continuously diffused through the porous tube into a combustion chamber where it forms a protective blanket on the inner surface of tube. This gaseous blanket substantially reduces the tendency of the titanium dioxide particles to adhere to the walls of the reactor. Since the combustion of titanium tetrachloride is an exothermic reaction, no provision is made to supply heat to the reaction mixture as it passes through tube. In fact, the U.S. Pat. No. 2,670,275 patent teaches that it is advantageous to remove heat from reactor chamber either by exposing the porous tube assembly to the atmosphere or by circulating a cooling fluid through a coil disposed about the porous tube.